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APPLICATION OF REMOTE SENSING (R.S.) ANDGEOGRAPHICAL INFORMATION SYSTEMS (G.I.S.) FOR FLOOD RISKASSESSMENT : A CASE STUDY OF AL KHARJ VALLEY AL KHARJ

SAUDI ARABIA

Dr. Khaled KHEDERAssistant Professor in Civil Engineering Department, College of Engineering, Salman bin Abdulaziz

University, P. O. Box 655, AlKharj 11942 Saudi Arabia.

E-mail : [email protected] , Mobile : 00966543663078

Abstract : During the last few years, many flood incidents have occurred in different regions of

the Kingdom of Saudi Arabia. These floods have caused serious damages over large areas

including loss of human lives and economical consequences for Saudi Arabia. The evaluation of

the floods constitutes the first step and the rational basis of management and mitigation measures

against flood damages. Remote Sensing (R.S.) and Geographical Information Systems (G.I.S.)

provide information that has proved useful for a wide range of application in disaster

management ([16], [17]). The application of R.S. and G.I.S. techniques for rapid flood mapping

and monitoring is an important tools of information for decision-makers ([3], [4]). Working

methods for flood management and mitigation have been developed in this paper by using Geo-Eye and SPOT-5 satellites images. These methods have been used to map the flood-affected

areas, to estimate the damages and design the hydraulic works needed. Data was collected from

various sources ([9]). A methodology for flood forecasting, planning and management with the

help of flood hazard maps for different return periods (25, 50 and 100 years) has been developed.

The population and physical vulnerabilities of the lowest area subjected to floods in the

downstream of Al Kharj Valley has been assessed and the necessary solution to avoid the flood

specially closed Al Kharj city has been proposed.

Keywords: Flood, G.I.S., R.S., Geo-Eye Satellite Images, Geo-processing, Damage assessment.
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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1 Introduction

The south of Al Riyadh experienced a flood during 2003 which affected the downstream

area of Al Kharj valley due to large quantity of water. Many difficulties have been index near

urban area close to the valley.In fact, Ministry of Water and Electricity of Saudi Arabia Kingdom attached a great

importance to fight against the unexpected flood in the future. There is need to explain the true

scenario by modeling flood ([10], [11]) in the downstream area of Al Kharj valley based on the

geospatial data from satellite images. Since flooding is the most frequent natural disaster, the

Ministry of Water and Electricity has been focusing its attention to Flood Hazard Mapping

(FHM) as one of the priority tasks to be accomplished in order to avoid the flood damage ([6],

[7]) close to the urban area at the downstream valley of Al Kharj.

Application of Geographical Information Systems (G.I.S.) and Remote Sensing (R.S.)

technology to map flood areas will make it easy to plan measures that may result in reducing the

flood damages and risks involved. Implementation of flood management program, that may

consist of flood forecasting, flood hazard and vulnerability mapping; may be very helpful in

saving precious human lives and property. Therefore, benefits of this project are very much

timely for inhabitants Al Kharj living on downstream side and in valley basin as well as for

governmental organizations. This paper focuses on analyzing the flood risk in the downstream

area, which is very close to the rapidly expanding urban planning area.

2 Study area

The study area is Al Kharj Valley basin in the south Al Riyadh region. This valley is

located in the western hill slopes of Al Kharj and being reached Al Hota in direction of Wadi Al

Dowasser about 100 km, which receives most of the south-west monsoon rainfall making the

Wadi network basin vulnerable for frequent floods. Al Kharj valley watershed sets cover about

5,000 km2

and major land-use covers are soil and shallow rocks mainly consisting of sand, siltysands, gravel and limestone as a bed rock. Moreover, substratum is also affected by underground

voids called karsts .

Geographically the basin lies between 663341 m and 750964 m E, and 2618055 m and

2693852 m N referring to W.G.S.-84 (World Geodetic System 1984) coordinate system and


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flows from a height of about 650 m above S.A.L.S. (Saudi Arabia leveling System). The figures

1 and 2 show the study area and different zooms highlight different components within the study

area.

a : Study area. b : Enlarged view of study area.

Figure 1 : The Study area - 2011.

a : Tributary Wadi to AlKharj valley. b : Enlarged view of wastewater basin .

Figure 2 : Wadi and waste water basin in the study area 2011.


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3 Methodology

The approach to this study involved data collection and geo-processing which are

required for Hazard analysis and Vulnerability analysis. Based on the Hazard analysis and

Vulnerability analysis, the Flood Risk analysis is carried out using Remote Sensing andGeographical Information Systems tools ([15]). In order to quantify the main variable used in the

simulation of flood risk, a Digital Elevation Model (D.E.M) has been performed using Remote

Sensing software (Erdas-2011) ([8], [19]) and the range of flood was estimated for return periods

of 25, 50 and 100 years. A G.I.S application (ArcGIS-9.3) ([1]) was used to propose necessary

solutions in the form of hydraulic works to stop the flood in the downstream area of Al Kharj

Valley, and to estimate the range of flood area especially close to the urban zones.

3.1 Hydrologic analysis

Hydrologic analysis consists of application of hydrologic model using HEC-HMS

(Hydrologic Modeling Software) ([5]). As availability of stream flow data was limited, a

calibrated rainfall - runoff model for the sub-basins based on HEC-HMS was used to predict

runoff for rainfall having 25, 50, and 100 years return period.

3.2 Hydraulic analysis

The drainage facilities should be designed for very rare storms to minimize the chance of

overflowing. Designing for this condition, however may result in very large facilities that might

result in raising the cost of the drainage facility. The decision on what frequency should be

selected for design purposes must therefore be based on a comparison of the capital cost for the

drainage facility and the cost to the public in case the urban area at the downstream is at risk by

storm runoff. Factors usually considered in making this decision include the importance of urban

area and its population density.

The drainage area is that area of land that contributes to the runoff at the point where the

open channel capacity is to be determined ([18]). This area is normally determined from a

topographic map or Digital Elevation Model (D.E.M.) after geo-processing satellite images using

R.S. software. The runoff coefficient, C, is the ratio of runoff to rainfall for the drainage area.


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The runoff coefficient depends on the type of ground cover, the slope of the drainage area, storm

duration and the slope of the ground. In cases where the drainage area consists of different

ground characteristics with different runoff coefficients, a representative value C w is computed

by determining the weighted coefficient using following relation ([21]) :

n

ii

n

iii

w

A

AC C

1

1

*

Where :

Cw : weighted runoff coefficient for the whole drainage area;

C i : runoff coefficient for watershed I;

A i : area of watershed I;

n : number of different watersheds in the drainage area.

Where :

Q : peak rate of runoff (m 3/s);

C : a coefficient representing the fraction of rainfall that remains on the surface of the

ground (runoff coefficient);

A : drainage area (acres, 1 acre = 4047 m 2);

I : average intensity for a selected frequency and duration equal to at least the time of

concentration (cm/h).

For any given storm, the rainfall intensity neither usually remains constant over a large

area, nor during the entire duration of the storm. The rational formula therefore uses the theory

that for a rainfall of average intensity, I, falling over an impervious area of size A, the maximumrate of runoff at the outlet to the drainage area Q, occurs when the whole drainage area is

contributing to the runoff and this runoff rate is constant. This requires that the storm duration be

at least equal to the time of concentration, which is the time required for the runoff to flow from


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the farthest point of the drainage area to the outlet. This condition is not always satisfied in

practice, particularly in large drainage areas.

The hydraulic design of drainage upon earth dams for a given storm entails the

determination of the minimum cross-sectional area of the hydraulic works that will accommodate

the flow due to that storm and prevent water from overflowing the sides of the hydraulic works.

The most commonly used formula for this purpose is Mannings formula, which assumes

uniform steady flow in the open channel and gives the mean velocity in the open channel as

following :

2

1

3

2

**1

S Rn

v

Where :

v : average discharge velocity (m/s);

R : mean hydraulic radius of flow in the open channel (m):

P a

R

a : channel cross-sectional area (m 2);

P : wetted perimeter (m);

S : longitudinal slope in open channel (m/m);

n : Mannings roughness coefficient.

The flow in the open channel is then given as following :

2

1

3

2

***1

* S Ran

avQ

Where :

Q : discharge a long an open channel (m 3/s).


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3.3 Topographical analysis

Based on the D.E.M modeling provided by R.S. software and referring to available

topographic maps of the study area, the figure 3 illustrates a long profile along the flood area in

which we observe at the middle of this flood area a basin where the water accumulates

temporarily. This situation can be easily shown in the figure 4 where a cross section within the

flood area has been taken. To confirm the existing basin at the middle of flood area we have

carried out three long profile and cross section (along axis of Wadi, one kilometer at right side

and one kilometer at left side of the flood area). This topographical analysis can be used to

estimate the quantity of water accumulated in the flood area.

Figure 3 : Longitudinal profile along the flood area.

Figure 4 : Cross section within the flood area.

A l t i t u

d e

( m )

Distance (m)

Axe of Wadi

Right side

Left side

A l t i t u

d e

( m )

Distance (m)

Middle of flood area

Rigth side

Left side


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3.4 Flood simulation using R. S.

The process of risk assessment of flood is summarized as follows :

Elaborate a geometric correction based on the accuracy image satellite; Make a mosaiking procedure to join satellite images;

Carry out a Digital Elevation Model (D.E.M) based on stereo images ([13], [14]) ;

Simulate the water layer by flowing it on the D.E.M ([20]) ;

Transfer simulated flow to geo-database and visualize it by ArcMap as vector layers.

3.5 Flood simulation using G.I.S.

To assess the risk of flood at the downstream area of Al Kharj Valley, a simulations using

G.I.S. software was performed. Summary of step is as shown below :

Digitization of the layers (buildings, Wadis, agriculture, wastewater, karsts, etc;

Building of a geo-database based on these layers ([2]);

Making of a geo-processing procedure to simulate the flood by intersecting layers;

Comparison of the results with the flood simulation given by R.S. software;

Proposing of the hydraulic solutions based on the HEC-Geo-RAS ([12]) .

4 - Results and interpretations

It was observed that depending on return periods of 10, 25, 50 and 100 years, the water

flow inundates the residential area (Figure 5). For 100-year return period the situation is

aggravated when the flood overruns a wide area of building close to the wadi. To quantify the

flooded area, the return periods data was transferred from Erdas 2011 to ArcMap-ArcInfo and a

geo-database was built to estimate the area flooded in each case. Figures 6 and 7 show theflooded area and total residential area that was flooded. We observed that the area covered by

flood can reach 2600 ha in which about 1600 ha falls in the residential area for return period of

100 years. This situation requires the designing of hydraulic solutions, taking into account the

hydrologic, hydraulic and topographical analysis results.


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a : Interface of Erdas Software. b : DEM of study area.

c : Water flow for 10 years return period. d : Water flow for 25 years return period .

e : Water flow for 50 years return period. f : Water flow for 100 years return period .

Figure 5 : Flood simulations using Erdas 2011 software.


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Figure 6 : Area of flood computed from geodatabase.

Figure 7 : Area of buildings vulnerable to flooding.

Referring to the geo-database, many data layers were over layered to indicate the most

hydraulic solution works in order to keep the floods away from the residential area. These layers

are the network of wadis, the agriculture, the wastewater, the karts, the sand dunes and the

buildings. Depending a return periods 25, 50 and 100 years, the water flow can reach the

residential area (Figure 8). Figures 8d, 8e and 8f show the hydraulic solution works to that can be

designed to monitor the quantity of water which can flow into two open channels along the

downstream valley especially close to the residential area. Table 1 and 2 indicate the dimensions

of the hydraulic works proposed and the quantities of flood volumes in the main Wadis of Al

A r e a o

f f l o o d

( h a

)

Return period (years)

A r e a o

f b u i l d i n g s

( h a

)

Return period (years)


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Kharj valley. Regarding to Geo-database from 2003 until 2011, we observed a decreasing trend

of agriculture area from about 34,000 ha to 10,000 ha. These hydraulic solutions allow for the

improvement of the agriculture activities close to it.

Table 9.1 : Designing hydraulic works refering to Geo-database : Earth Dams.

Watershed name Max. area ofbasin(m 2)

Max. depth ofwater (m)

Length ofdam (m)

Max. Flowout (m 3/s)

Al Sout Wadi 3434395 1.5 250 165Al Kharj Wadi 3138831 3.0 400 415

Table 9.2 : Designing hydraulic works refering to Geo-database : Open Channels.(O. C.: Open Channel)

Watershed name Volume of flood(m 3)

Type and form ofhydraulic works

Flow depth (m)

Al Sout Wadi 4820760 O. C.Trapezoidal

4

Al Kharj Wadi 9641520 O. C.Trapezoidal

6

Al Kharj Valley 14462280 O. C.Trapezoidal

7

a : Area flood for 25 years. b : Area flood for 50 years.


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